Reorganize services/ into category subfolders

Group service modules by category (media, network, vpn, identity,
dev, desktop) to make the growing services/ dir easier to navigate.
containers.nix stays at the top level since it's a shared backend,
not a single-category service.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

services/media/audiobookshelf.nix

@@ -0,0 +1,16 @@
+{ ... }:
+
+# Audiobookshelf audiobook/podcast server.
+# Listens on all interfaces: :8000 stays closed on the LAN (no openFirewall),
+# but reachable over the trusted tailscale0 interface and via localhost (caddy).
+# Library/media paths are set in the web UI — point them at /mnt/data/...
+# Runs as user `audiobookshelf`; added to `users` so it can read group-owned
+# library dirs on the RAID.
+{
+  services.audiobookshelf = {
+    enable = true;
+    host = "0.0.0.0";
+    port = 8000;
+  };
+  users.users.audiobookshelf.extraGroups = [ "users" ];
+}